To use it, you'll need a Moz Access ID and Secret. You can get those for free by signing up for the Moz API at https://moz.com/products/api
Once you have your Access ID and Secret, use the Settings (the "gear" icon in the upper right) to enter them so that the system can access the Moz API.
The app is divided into two tabs: The "Manage Links" tab an the "Check Metrics" tab.
To enter links that you want to check, use the "Manage Links" tab. Simply enter the URL at the top and click "Add". If you want to delete a link that you've previously added, just click the trash can icon to the right of the URL. Please note that you only have to enter URLs once. The system will save them so that you don't have to enter them every time you restart the app.
To check the metrics for your URLs, click on the "Check Metrics" tab. You can select either the "Page to Domain" or "Page to Page" scopes. The "Page to Domain" scope will check for backlinks to the domains of the URLs you entered. The "Page to Page" scope will check for backlinks to the specific URL.
Once you click "Check" on the "Check Metrics" page, the system will use your Moz Access ID and Secret to check backlinks to the URLs you specified. That might take a few minutes, depending on how many links you entered. The app will show you the DA, PA, and backlinks for each of your links.